Discharge lamps, such as CCFL, are widely used as backlight in various electronic devices. Inverter circuits that convert relatively low DC voltages into high frequency AC voltages are generally required for driving discharge lamps. In addition, during an ignition process, a striking voltage of about three times of that in normal operation condition is needed to ignite discharge lamps. Therefore, a control circuit is often needed for regulating the inverter circuit to provide stable AC voltages during normal operation and high voltages during the ignition process.
The control circuit often regulates a duty ratio of a switching signal while maintaining the switching frequency at a constant working frequency so that the inverter outputs a stable AC voltage to drive a discharge lamp under normal operation conditions. During ignition processes or under fault conditions (e.g., open or short lamp conditions), the control circuit sweeps the switching frequency high until a striking frequency is reached so that the inverter outputs a higher AC voltage (commonly referred to as a striking voltage) to get the discharge lamp ignited.
Typically, the control circuit provides separate control signals for regulating the duty ratio of the switching signal and the switching frequency of the inverter, respectively. FIG. 1 is a block diagram illustrating a prior art control circuit 100 for a CCFL inverter. As shown in FIG. 1, the control circuit 100 includes a feedback network 101 for detecting a lamp current or a lamp voltage and generating a feedback signal; a duty control signal generator 103 for receiving the feedback signal and generating a duty control signal based on the feedback signal; a frequency control signal generator 105 for detecting the operation status of the lamp and generating a frequency control signal when the lamp is in ignition or in fault; a duty control module 107 for receiving the duty control signal and generating a duty signal regulated by the duty control signal; a frequency control module 109 for receiving the frequency control signal, generating a frequency signal with a constant frequency when the lamp is in normal operation, and sweeping the frequency of the frequency signal high to a striking frequency when the lamp is in ignition or in fault; a driving module 111 for receiving the duty signal and the frequency signal and generating a switching control signal to drive the switching devices of the lamp inverter in ON and OFF states so that the lamp inverter provides appropriate driving voltage to the lamp.
As a result, the prior art control circuit 100 requires a large number of electrical elements for implementing the various circuitry. Further, when the integrated circuit chip is packaged, separate pins are needed for external connections to realize the control loop compensation, the frequency sweeping rate, and striking frequency setting.